The UN has expressed outrage at a deadly attack on one of its bases in South Sudan, saying it could "constitute a war crime", a report said. Thursday's attack by armed youths on the base in Bor left at least 58 dead, including children, BBC reported. Thousands of civilians are sheltering from ethnic violence there. The UN said its peacekeepers returned fire as a mob of some 300 people forced their way into the base in an "unprovoked attack". Thousands of people have been killed in South Sudan since fighting began in December between supporters of President Salva Kiir and his former deputy, Riek Machar. More than one million people have fled their homes in the conflict, some to neighboring countries.